Olefin polymerization catalyst systems may be divided into Ziegler-Natta and metallocene catalysts, and these highly active catalyst systems have been developed in accordance with their characteristics. Ziegler-Natta catalyst has been widely applied to commercial processes since it was developed in the 1950's. However, since the Ziegler-Natta catalyst is a multi-site catalyst in which a plurality of active sites are mixed, it has a feature that a resulting polymer has a broad molecular weight distribution. Also, since a compositional distribution of comonomers is not uniform, there is a problem that it is difficult to obtain desired physical properties.
Meanwhile, the metallocene catalyst includes a main catalyst having a transition metal compound as a main component and an organometallic compound cocatalyst having aluminum as a main component. Such a catalyst is a single-site catalyst which is a homogeneous complex catalyst, and offers a polymer having a narrow molecular weight distribution and a uniform compositional distribution of comonomers, due to the single site characteristics. The stereoregularity, copolymerization characteristics, molecular weight, crystallinity, etc. of the resulting polymer may be controlled by changing a ligand structure of the catalyst and polymerization conditions.
U.S. Pat. No. 5,032,562 discloses a method for preparing a polymerization catalyst by supporting two different transition metal catalysts on one support. This catalyst is prepared by supporting a titanium (Ti)-based Ziegler-Natta catalyst which produces a high molecular weight polymer and a zirconium (Zr)-based metallocene catalyst which produces a low molecular weight polymer on one support, and results in a bimodal molecular weight distribution. This catalyst has disadvantage in that the supporting procedure is complicated and morphology of polymers is poor due to a cocatalyst.
U.S. Pat. No. 5,525,678 discloses a method for using a catalyst system for olefin polymerization, in which a metallocene compound and a non-metallocene compound are simultaneously supported on a support to conduct simultaneous polymerization of a high molecular weight polymer and a low molecular weight polymer. However, there are disadvantages that the metallocene compound and non-metallocene compound must be separately supported and the support must be pretreated with various compounds for supporting.
U.S. Pat. No. 5,914,289 discloses a method for controlling a molecular weight and a molecular weight distribution of polymers using metallocene catalysts which are respectively supported on supports. However, a large amount of solvent and a long period of time are required to prepare the supported catalysts, and a process of supporting metallocene catalysts on the respective supports is troublesome.
Korean Patent Application No. 2003-12308 discloses a method for controlling molecular weight distributions of polymers, in which the polymerization is performed while changing a combination of catalysts in a reactor by supporting a dinuclear metallocene catalyst and a mononuclear metallocene catalyst on a support together with an activating agent. However, this method has limitations in simultaneously realizing the characteristics of respective catalysts. In addition, there is a disadvantage that the metallocene catalysts are departed from a supported component of the resulting catalyst to cause fouling in the reactor.
Therefore, to solve the above drawbacks, there is a continuous demand for a method for preparing polyolefins with desired physical properties by easily preparing a hybrid supported metallocene catalyst having an excellent activity.